The investigation of the major determinants of monoamine synthesis and turnover in vivo is of scientific interest because monoamine levels in the central nervous system (CNS) play critical roles in neuropsychiatric, neuroendocrine and cardiovascular diseases. Tyrosine and tryptophan hydroxylase are known to be the rate-limiting steps in the syntheses of catecholamines and serotonin, respectively. Current evidence suggests that the in vivo rate of synthesis of these compounds may be mediated by the concentration of reduced biopterin (BH4). Recent reports in the literature have indicated a high correlation between BH4 levels and tyrosine hydroxylase activity in selected brain areas. Our preliminary results indicate significant amounts of BH4 in brain areas known to contain large amounts of tyrosine and tryptophan hydroxylase. We have reported that BH4 content correlates well with both tyrosine and tryptophan hydroxylase activities measured in 10 discrete rat brain areas. An inordinately high content of reduced cofactor is present in the hypothalamus, pituitary, and pineal gland based on the amount of hydroxylase enzyme activity in these same tissues. The presence of unusually high amounts of BH4 in these "neuroendocrine" type tissues may indicate another role for BH4.